Conventional fiber optic rotation sensors have been known as Sagnac gyroscopes in which a light emitted from a light source is divided into two lights to be propagated through an optical fiber sensing loop in clockwise and counter-clockwise directions, respectively, the two lights are coupled after the propagation to provide interference light outputs to be supplied to a light receiver for converting light signal to electric signal, and the converted electric signal is processed in a logic circuit to calculate an angular velocity.
In the conventional fiber optic sensor, a relativistic inertia effect to be defined "Sagnac effect" in which a propagation time difference of the two lights propagated through the optical fiber sensing loop in the opposite directions, that is, a phase difference of the two lights is proportional to an angular velocity of the optical fiber sensing loop is utilized. In practical fiber optic sensors, the phase difference is not directly detected, but indirectly detected in a change of light outputs, as defined by an equation (1). EQU Ps=Ps.sub.1 +Ps.sub.2 +2Ps.sub.1 Ps.sub.2 cos.DELTA..phi. (1)
where Ps is a light output value, Ps.sub.1 and Ps.sub.2 are clockwise and counter-clockwise directional components of the light output value, respectively, and .DELTA..phi. is a phase difference.
A relation between the phase difference .DELTA..phi. and an angular velocity (deg/sec) is defined by an equation (2). EQU .DELTA..phi.=(8.pi.S/.lambda.C).times.Q (2)
where S is an area of the optical fiber sensing loop, .lambda. is a wavelength of light, C is a velocity of light and .OMEGA. is an angular velocity.
As understood from the equation (1), the light output value Ps is proportional to cos .DELTA..phi..Consequently, a sensitivity of the fiber optic rotation sensor becomes low, when a rotation is very small. For this reason, a minute angular velocity can not be precisely detected. For the purpose of improving this disadvantage, a phase bias method in which a phase modulator consisting of, for instance, piezo-electric ceramic is utilized is adopted to provide the phase difference .DELTA..phi. of the clockwise and counter-clockwise propagated lights with a shift of .pi./2. Thus, an equation (3) is defined. ##EQU1##
In the fiber optic rotation sensor, a logic circuit includes a memory having a correlation map storing angular velocities relative to light output values, so that an angular velocity is read from the correlation map of the memory by accessing an address thereof determined by a detected light output.
In the fiber optic rotation sensor, an angle of the optical fiber sensing loop is detected in addition to the detection of an angular velocity. For this purpose, an angular velocity is integrated by time, so that an angle of the optical fiber sensing loop is obtained. This allows the fiber optic rotation sensor to be applied to a navigation system of a vehicle, a robot of a transportation system, etc.
However, the conventional fiber optic rotation sensor has a disadvantage in that the precision of detecting an angular velocity is not expected to be high, because optical parts such as an optical fiber, etc. used therein is affected in its light transmission property by the change of temperature, so that the dispersion of wavelength occurs to result in the temperature dependency of light output values. In other words, the correlation map included in the memory is prepared without considering the temperature dependency of the light transmission property.